1. Field of the Invention
The present invention relates to non-asbestos friction materials which are particularly well suited for use as brake linings in drum brakes because they undergo little wear during high-speed braking and have an excellent heat resistance, a stable braking performance and good strength.
2. Prior Art
Drum brake friction materials used for braking in automobiles, large trucks, railroad cars and various industrial equipment are required to have a number of performance features. These features include not only an excellent braking effectiveness (i.e., a high friction coefficient), but also a high heat resistance, minimal noise generation, and good strength to prevent cracks from forming when the friction material is bolted or riveted to the brake shoe.
Unlike in disk brakes, the brake shoe and brake lining in drum brakes are situated at the interior of the drum, making it difficult for heat generated during braking to dissipate to the exterior. As a result, the brake shoe and the friction material at the surface of the brake lining are scorched by heat, which greatly reduces braking effectiveness and tends to lead to brake fade. Accordingly, there is a strong desire for better heat resistance.
Moreover, the development of extensive highway networks and the increased use of long-distance transportation in recent years have brought about a sharp rise in the frequency of high-speed braking, greatly increasing both the amount of wear by the brake linings and, owing to the thermal history, the frequency of cracking at the surface of the lining against which the brake drum rubs.
Various attempts have been made to resolve these problems in frictional materials for drum brakes. Such efforts have included adding a large amount of graphite as a lubricant to reduce noise (in which case the amount of binder also increases), enhancing heat resistance by adding cashew dust that discharges little tar when heated, and including a higher proportion of binder to improve the strength of the friction material.
However, none of these approaches has succeeded in providing friction materials for drum brakes with all the desired performance features. Indeed, the prospects for developing in particular a friction material for drum brakes endowed with low wear during high-speed braking, stable brake effectiveness, and excellent heat resistance have been regarded as very poor.
Therefore, the object of the present invention is to provide a high-quality non-asbestos friction material having low wear during high-speed braking, excellent heat resistance, stable braking effectiveness and good strength.
The inventor has found that, in non-asbestos friction materials, incorporating high-tar cashew dust and lowering the amount of binder and graphite are effective ways for achieving reduced wear during high-speed braking, improved heat resistance and other desirable aims.
That is, in non-asbestos friction materials obtained by molding and curing a composition comprising a fibrous base, a filler and a binder, the incorporation of a high-tar cashew dust as at least part of the filler results in the formation by the high-tar cashew dust of a thin, uniform organic film on the friction surface, thereby reducing the amount of friction material wear during high-speed braking and stabilizing braking effectiveness. Also, by having the total amount V1 of binder present in the friction material composition and the amount V2 of graphite present as part of the filler (both amounts being expressed in volume percent based on the overall composition) be such that the product V1xc3x97V2 is from 10 to 100 inclusive and the sum V1+V2 is from 10 to 30 inclusive, the level of organic components can be held much lower than in the prior art. This makes it possible to achieve a high-quality non-asbestos friction material having a greatly improved heat resistance and, together with this, an excellent performance during high-speed braking.
The inventor has also found that, in non-asbestos friction materials comprising a molded and cured composition that includes a fibrous base, a filler and a binder, when the filler is at least partially composed of 1 to 35 vol % of high-tar cashew dust and less than 5 vol % of graphite and when the binder is present in a total amount of at most 30 vol %, based on the overall composition, these components work together to lower the amount of wear. As a result, in spite of the presence of much lower amounts of graphite and binder than in prior-art non-asbestos friction materials, or rather because of the presence of graphite and binder in such small amounts, the friction material has a vastly improved heat resistance. Hence, there can be obtained a high-quality non-asbestos friction material having low wear during high-speed braking, excellent heat resistance, stable braking effectiveness and sufficient strength to prevent crack formation when the friction material is bolted or otherwise mounted on a brake shoe.
Therefore, according to a first aspect, the invention provides a non-asbestos friction material obtained by molding and curing a composition comprising a fibrous base, a filler and a binder; wherein the binder is present in a total amount V1 and the filler includes in part graphite present in an amount V2, each expressed in volume percent of the overall composition, such that the product V1xc3x97V2 is from 10 to 100 inclusive and the sum V1+V2 is from 10 to 30 inclusive.
According to a second aspect, the invention provides a non-asbestos friction material obtained by molding and curing a composition comprising a fibrous base, a filler and a binder; wherein the filler is at least partially composed of 1 to 35 vol % of high-tar cashew dust and less than 5 vol % of graphite, based on the overall composition, and the binder is present in a total amount of at most 30 vol %.
The non-asbestos friction material of the invention is made by molding and curing a non-asbestos friction material composition composed primarily of a fibrous base, a filler and a binder. According to a first embodiment of the invention, the friction material includes the binder in a total amount V1 and includes as part of the filler graphite in an amount V2 (both amounts being expressed in percent by volume of the overall composition), such that the product V1xc3x97V2 is from 10 to 100 inclusive and the sum V1+V2 is from 10 to 30 inclusive (that is, 10xe2x89xa6V1xc3x97V2xe2x89xa6100 and 10xe2x89xa6V1+V2xe2x89xa630). By satisfying these conditions, the inventive material has a vastly improved performance during high-speed braking.
The product V1xc3x97V2 is in a range of preferably 15 to 100, more preferably 20 to 90, and most preferably 20 to 80. The sum V1+V2, which represents the combined amount of binder and graphite, is in a range of preferably 10 to 27, more preferably 15 to 25, and most preferably 17 to 28.
If the total amount of binder V1 and the amount of graphite V2 (both in volume percent) are such that the product V1xc3x97V2 and the sum V1+V2 do not fall within the above ranges, the amount of organic components in the friction material becomes too large, lowering the heat resistance and compromising the performance during high-speed braking.
The binder may be any known binder commonly used in friction materials. Illustrative examples include phenolic resins, various rubber-modified phenolic resins such as high-ortho phenolic resins modified with acrylonitrile-butadiene rubber (NBR), NBR-modified phenolic resins and acrylic rubber-modified phenolic resins, and also melamine resins, epoxy resins, NBR, nitrile rubber and acrylic rubber. Any one or combinations of two or more of these may be used. The binder is included in a total amount V1 which is up to 30 vol %, preferably up to 27 vol %, more preferably up to 25 vol %, even more preferably up to 23 vol %, and most preferably up to 20 vol %, based on the overall amount of the friction material composition. The amount of binder preferably has a lower limit of at least 5 vol %. Too large a total amount V1 of the binder excessively increases the organic components within the friction material, lowering the heat resistance.
The graphite used in the non-asbestos friction material of the invention may be any known graphite commonly used in friction materials. The graphite may be a naturally produced or synthetic graphite, and may be in any form, such as scales, needles or spheres. The graphite has an average particle size of preferably 30 to 1,000 xcexcm, and especially 50 to 500 xcexcm.
The graphite is included in an amount V2, based on the overall friction material composition, of less than 5 vol %, preferably from 1 vol % to less than 5 vol %, more preferably within a range of 1 to 4 vol %, and most preferably within a range of 1 to 3 vol %. The addition of too much graphite results in an excessive amount of organic components within the friction material, lowering the heat resistance of the friction material.
According to a second embodiment of the invention, the friction material, which is typically used for drum brakes, includes as at least part of the filler a high-tar cashew dust in an amount of 1 to 35 vol %, preferably 5 to 30 vol %, more preferably 5 to 25 vol %, and most preferably 10 to 25 vol %, based on the overall friction material composition. If the amount of high-tar cashew dust is outside the above range, the inventive friction material having a low amount of wear during high-speed braking and stable braking effectiveness cannot be achieved.
The high-tar cashew dust has a tar content of preferably at least 10 wt %, more preferably 10 to 15 wt %, and most preferably 10 to 13 wt %. Too low a tar content fails to result in the formation of a thin, uniform film on the friction surface, preventing the intended objects and advantages of the invention from being achieved. xe2x80x9cTar content,xe2x80x9d as used herein, refers to the weight percent of tar in the cashew dust, as measured by extraction with acetone. The tar content of the high-tar cashew dust used in the invention is significantly higher than the tar content of conventional cashew dust, which is typically 6 to 8 wt %. The high-tar cashew dust has an average particle size of preferably 50 to 500 xcexcm, and especially 200 to 500 xcexcm.
In addition to the above-described high-tar cashew dust and graphite, the friction material of the invention may include also other known organic or inorganic fillers commonly used in friction materials. Illustrative examples include molybdenum disulfide, antimony trisulfide, calcium carbonate, precipitated calcium carbonate, barium sulfate, magnesium oxide, calcium hydroxide, calcium fluoride, slaked lime, talc, molybdenum trioxide, antimony trioxide, zirconium silicate, iron oxide, mica, iron sulfide, zirconium oxide, metal powder, fused silica, silicon dioxide, alumina, chromium oxide, vermiculite, ground tire rubber, rubber dust (rubber powder and granules), nitrile rubber dust (vulcanized product), and acrylic rubber dust (vulcanized product). These may be used alone or as combinations of two or more thereof.
The amount of such fillers other than high-tar cashew dust and graphite is preferably from 10 to 60 vol %, and especially from 25 to 50 vol %, based on the overall frictional material composition.
The fibrous base may be any organic fiber or inorganic fiber other than asbestos that is commonly used in friction materials. Illustrative examples of suitable materials include inorganic fibers such as metal fibers (e.g., iron, copper, brass, bronze, and aluminum), ceramic fibers, potassium titanate fibers, glass fibers, carbon fibers, rock wool, wollastonite, sepiolite, attapulgite, and man-made mineral fibers; and organic fibers such as aramid fibers, polyimide fibers, polyamide fibers, phenolic fibers, cellulose, and acrylic fibers. Any one or combination of two or more of these may be used.
The fibrous base may be used in the form of short fibers or a powder. It is added in an amount of preferably 5 to 30 vol %, and most preferably 10 to 20 vol %, based on the overall friction material composition.
The method of making the non-asbestos friction material of the invention involves uniformly blending the above-described fibrous base, fillers and binder in a suitable mixer such as a Henschel mixer, Loedige mixer or Eirich mixer, and preforming the blend in a mold. The preform is then molded at a temperature of 130 to 200xc2x0 C. and a pressure of 100 to 1,000 kg/cm2 for a period of 2 to 15 minutes. The resulting molded article is postcured by heat-treating at 140 to 250xc2x0 C. for 2 to 48 hours, then spray-painted, baked and ground as needed, giving the finished article.
In the case of automotive brake linings, production may be carried out by known methods.
The non-asbestos friction materials of the invention can be used in disk brakes or drum brakes, and are highly suitable for a variety of related applications, including disk pads, brake shoes and brake linings in automobiles, large trucks, railroad cars and various types of industrial equipment. They are particularly well-suited to use as brake linings in drum brakes.